The invention relates to a picture display device comprising a cathode ray tube having a display window with a display screen, said display screen being provided with at least two index electrodes, each index electrode comprising a plurality of conducting elements extending substantially parallel to an axis of symmetry of the display screen.
Picture display devices comprising such index electrodes are known from GB 2122415 and are usually referred to as xe2x80x98indexxe2x80x99 display devices.
In such known index display devices, the electron beamxe2x80x94when impinging on a conducting element of an index electrodexe2x80x94generates an index signal which is indicative of the position of the electron beam with respect to said conducting element and/or of the shape of the electron beam. The index signals of the index electrodes are measured and subsequently used in a control loop acting on the deflection and/or formation of the electron beam in order to correct the trajectory and/or shape of the electron beam when it deviates from its nominal trajectory and/or shape.
Although such known index devices work satisfactorily in many circumstances, there is a wish to control the electron beam position and/or shape more accurately.
It is an object of the invention to provide an index display device with improved index signals from the index electrodes.
To this end, the display device in accordance with the invention is characterized in that at least one index electrode comprises at least one conducting element, one end of which is connected by means of a cross-connection to an opposite side end of another conducting element of said at least one index electrode.
The deflection system of the cathode ray tube indeed generates considerable magnetic fields which induce parasitic currentsxe2x80x94also called crosstalk currentsxe2x80x94in the conducting elements of the index electrodes. A deflection system within the scope of the invention includes the detection unit as such, but also auxiliary deflection means, for instance, for fine-tuning the deflection. In fact, in index tubes the systems used for tracking the beams (and thus for improving the position and or shape of the electron beams) themselves are a cause of crosstalk currents. These crosstalk currents add up to the index signals and thus constitute noise for the measurement circuit because they do not comprise any information pertaining to the position and/or shape of the electron beam. All of these crosstalk currents flow in the same direction in the conductive elements of an index electrode, so that said crosstalk currents at least partly compensate each other by cross-connecting two conducting elements and thereby reduce the noise.
Conversely, at any given moment, the electron beam only impinges on one conducting element of an index electrode, so that no reduction of the index signal is introduced by cross-connecting conducting elements. The signal/noise ratio is consequently increased, which allows improvement of the accuracy of the electron beam position and/or shape control.
When an index electrode comprises more than two conducting elements, the above-described compensation effect of crosstalk currents can be increased by cross-connecting additional conducting elements. In order to reduce the number of cross-connections, it is advantageous in such a case to cross-connect groups of conducting elements, one group comprising a first set of conducting elements connected together at their same side ends, another group comprising another set of conducting elements connected together at their opposite side ends, while both groups comprise substantially the same number of conducting elements.
Since the magnetic field of the deflection system at the surface of the display screen is generally symmetric with regard to an axis of symmetry of the display screen, it is also advantageous to have both groups of conducting elements disposed symmetrically with regard to said axis of symmetry. In doing so, the sum of the crosstalk currents induced in the conducting elements of the first group will almost equal the sum of the crosstalk currents induced in the conductive elements of the second group, resulting in an almost total compensation of both sums when cross-connecting both groups.
In a simple version, the index electrode is divided into two groups of conducting elements, the first group comprising the conducting elements located on one side of the axis of symmetry of the display screen, the second group comprising the conducting elements located on the opposite side of said axis of symmetry, while both groups are symmetrically disposed with regard to said axis of symmetry and are cross-connected by their opposite side ends.
An index display device often comprises more than one index electrode. Evidently, it is also possible to apply the structure and the interconnections of conducting elements as described above to any other index electrode, thereby improving the signal/noise ratio of the index signal from said any other electrode as well.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.